It is proposed to continue and expand studies on the epidemiology of type B hepatitis being currently conducted in a rural population of 45,000 in Costa Rica, where both forms of the disease occur endemically in the absence of parenteral mechanisms of transmission. Through surveillance and case finding, the epidemiological patterns of occurrence and mode of spread of type B hepatitis will be studied, with emphasis on the definition of mechanisms of transmission and the role that chronic HBsAg carriers play in the dissemination of the disease. Comparative data will be obtained for type A hepatitis. The role of body fluids, especially saliva, as vehicles of transmission of type B hepatitis will be investigated. The infectivity potential of HBsAg carriers and acute hepatitis cases will be defined. A large number of specimens accumulated in our serum bank will be examined for antibodies to hepatitis A using newly developed serologic tests to define immunity patterns in the population and correlate them with epidemiological data on hand. In addition studies will be conducted on the clinical characteristics of both types of disease, including the investigation of immunoglobulin responses in typical and atypical hepatitis.